NMR investigation of the heme electronic structure in deoxymyoglobin possessing a fluorinated heme

The heme electronic structures of deoxymyoglobins (deoxy-Mbs) reconstituted with 13,17bis(2-carboxylatoethyl)-3,8-diethyl-2,12,18-trimethyl-7-(trifluoromethyl)porphyrinatoiron (III) (7-PF), 13,17-bis(2-carboxylatoethyl)-3,7-difluoro-2,8,12,18tetramethylporphyrinatoiron(III) (3,7-DF), and 13,17-bis(2-carboxylatoethyl)-3,8-diethyl2-fluoro-7,12,18-trimethylporphyrinatoiron(III) (2-MF) have been characterized by 1H and 19F NMR. The analysis of heme methyl proton shift patterns of the hemes in their bis-cyano forms demonstrated that, owing to the substitution of a strongly electron-withdrawing perfluoromethyl group, CF3, to porphyrin, the porphyrin system of 7-PF is more significantly distorted from four-fold symmetry than those of the ring-fluorinated hemes, 3,7-DF and 2-MF. The presence of the heme orientation disorder resulted in the observation of the two well-resolved 19F signals in the spectra of deoxy-Mbs possessing 7-PF and 2-MF. The 19F signals of deoxy-Mb possessing 7-PF exhibited a relatively large difference in paramagnetic shift (~30 ppm), despite their small paramagnetic shifts (~30 ppm), supporting the significant contribution of a spin delocalization mechanism in this Mb due to the d-electron configuration derived from the 5E ground state. On the other hand, 19F signals of deoxy-Mbs with 3,7-DF as well as 2-MF exhibited large paramagnetic shifts (~250 ppm) with a relatively small difference in the paramagnetic shift (~20 ppm), indicating the predominant contribution of spin delocalization, due to a d-electron configuration derived from the 5B2 ground state. These results demonstrate for the first time that the relative contributions of the orbital ground states derived from 5E and 5B2 states to the heme electronic structure in deoxy-Mb are affected by the distortion of the porphyrin system exerted by chemical properties of the heme peripheral side-chains.